1. Field of the Invention
The present invention relates to a toner cartridge provided in an image forming apparatus and, more specifically, to a toner cartridge, a developing device and an image forming apparatus allowing accurate control of toner replenish amount.
2. Description of the Background Art
An image forming apparatus utilizing static electro-photographic system generally forms an image on a sheet of recording paper through charging, exposing, developing, transferring, fixing, cleaning and neutralizing steps. At the charging step, a surface of a photoreceptor drum, which is driven to rotate, is uniformly charged by a charging device. At the exposure step, the charged surface of photoreceptor drum is irradiated with laser beam by an exposure device, whereby an electrostatic latent image corresponding to image information is formed. At the developing step, the electrostatic latent image formed on the surface of photoreceptor drum is developed by a developing device and a toner image is formed. At the transferring step, the toner image formed on the surface of photoreceptor drum is transferred to the sheet of recording paper by a transferring device. At the fixing step, the transferred toner image is fixed by a fixing device on the sheet of recording paper, by the application of heat and pressure. At the cleaning step, toner left on the surface of photoreceptor drum after transfer of the toner image is removed by a cleaning device, so that the surface of photoreceptor drum is cleaned. The toner removed at this time is recovered to a prescribed recovering unit. At the neutralizing step, charges left on the cleaned surface of photoreceptor drum are removed by a neutralizer, to be ready for the next image formation.
In the image forming apparatus as such, the electrostatic latent image is developed, using as the developer, a one-component developer consisting only of toner, or a two-component developer consisting of toner and carrier. Use of one-component developer is advantageous in that a stirring mechanism for uniformly mixing toner and carrier is unnecessary and, therefore, the developing device comes to have a simple structure and hence the developing device can be made smaller, whereas it is disadvantageous in that amount of toner charges tends to be unstable. Use of the two-component developer is disadvantageous in that a stirring mechanism for uniformly mixing toner and carrier is necessary and, therefore, the developing device comes to have a complicated structure and the developing device comes to have a larger size, while it is advantageous in that amount of toner charges is stable and well suited for use in a high-speed machine. Therefore, the two-component developer is often used in a high-speed image forming apparatus and a color image forming apparatus.
Recently, in order to meet users' demand for higher image quality and low energy consumption, toner of small grain size having low softening temperature and volume-average grain size of about 5 μm to about 9 μm is often used. The toner of small grain size allows fixing at a low temperature, and well attains higher resolution and lower feeling of granularity, leading to higher image quality.
The toner in developer is consumed at every developing operation. Therefore, a toner cartridge is provided in the developing device, for replenishing toner to the developing device in accordance with the consumed amount of toner. In a conventional toner cartridge, a toner outlet is formed at one end of a toner discharging member for carrying the toner to the toner outlet and, therefore, much toner tends to be accumulated at said one end of the toner discharging member, that is, the terminating end of toner carrying direction, resulting in a problem that toner clogs. Further, since the toner is carried in one direction, excessive load tends to be imposed on the toner on the terminating end of toner carrying direction and, in addition, there is a stress caused by friction with the toner discharging member. As a result, agglomeration of toner possibly occurs, or external additive to the toner undesirably enters inside the toner, so that toner characteristics tend to deteriorate.
Referring to FIG. 1, by way of example, Japanese Patent Laying-Open No. 2001-83802 (hereinafter referred to as “'802 application”) discloses a toner cartridge 13 as a solution to the above-described problem, in which a toner outlet 15 is provided on the side of a central portion in the longitudinal direction of a first stirring and carrying member 17 as the toner discharging member, and a toner carrying part 19 is formed on the first stirring and carrying member 17 such that toner carrying direction is reversed substantially at the center in the longitudinal direction thereof, whereby the toner comes to be carried to the central portion in the longitudinal direction of first stirring and carrying member 17.
Further, Japanese Patent Laying-Open No. 63-18380 (hereinafter referred to as “'380 application”) discloses a toner cartridge formed such that the volume near the toner outlet is made smaller than the volume of other portions.
While the toner of small grain size has superior characteristics to attain higher image quality and realizes fixing at a low temperature, it has low fluidity. Therefore, it is difficult by the conventional toner cartridge such as disclosed in '802 and '380 applications to control amount of toner to be replenished to the developing device. Particularly, it is difficult to accurately control the amount of toner replenishment, as an accidental toner leakage occurs while toner is not replenished.